Verify overrides against chain metadata; per-para cores and messaging switches

[mordamax]

• Storage keys derived from pallet/item names instead of hardcoded hashes; items the runtime doesn't have are skipped (e.g. ValidatorSet::Validators, which Polkadot/Kusama have no pallet for, was previously written blindly).
• Every override value is decoded against its real on-chain type and must re-encode byte-identically; trailing bytes are an error, which is what the ValidatorGroups bug looked like.
• Configuration::ActiveConfig is read live and patched (num_cores only) instead of replaced, so executor_params, async-backing params and max_pov_size of the bitten chain survive.
• --para-cores <id>=<n> and --keep-messaging-state (both also settable in the TOML config).

Verification needs an RPC endpoint: the relay always has one, a parachain only when rpc_endpoint is set — otherwise overrides fall back to unverified with a warning.

Closes #124
Closes #125